Line spread function (LSF) of a picosecond streak tube mainly depends on photoelectrons initial velocity distribution and on the time dispersion of electrons. An experimental tech-nique for direct LSF definition based on application of subpicosecond light pulses from Rho-damine-6G dye laser is developed. The obtained experimental results make it possible to construct a more reliable model for streak image tube behaviour in subpicosecond temporal range.